The present invention relates to a process for effectively preparing L-ribose from a 1,4-lactone compound.
L-ribose is widely used as raw materials for food, cosmetics or medicines. Recently, as physiological functions of L-nucleosides have been gradually discovered with the support of genetic engineering, new medicines containing L-nucleoside as active ingredient are newly developed and thus, the demand for L-nucleosides are on the increase. Particularly, demand for L-ribose is highly increased as it is the key intermediate for BW1263w94(Glaxo Wellcome) as antiherpes and L-FMAU (Bukwang and Triangle) as antihepatitis B. Thus, many researchers in this field are interested in the development of an industrially applicable process for preparing the same (see: Nucleic acid and Nucleotide 18(2), 187(1999); JP 11/12294; WO 98/39347). In this regard, recently, there has been developed a process for preparing L-ribose and rare sugars using an enzyme controlling the chirality (see: WO 99/61648).
However, since the existing processes for the preparation of L-ribose had the problems of low production yield and productivity, a new effective process was strongly required. Complying with such a requirement, more effective chemical processes have been filed and patented. The researches were much more activated recently, and as a result thereof, even a biological method using an enzyme has come.
Hitherto, L-ribose of the following formula (1) was conventionally prepared from L-arabinose of the following formula (2). That is, the hydroxy group at C2 position of L-arabinose was stereoselectively converted to give L-ribose. 
One of the methods for providing L-ribose from L-arabinose proceeds via L-arabinal intermediate as depicted in the following Scheme 1 (see: J. Am. Chem. Soc., 56, 1152 (1934)). However, this method has some problems in economy due to the low yield: 
Further, as depicted in the following Scheme 2, the C2 position of L-arabinose may be oxidized by pyridine dichromate and then reduced by sodium borohydride (NaBH4) to provide L-ribose in a considerably high yield. However, since it is difficult to handle pyridine dichromate, said method can hardly be applied to an industrial production (see: Nucleosides and Nucleotides, 18(2), 187(1999)). 
Recently, a method wherein C2 position is stereoselectively converted in a direct manner using molybdenium trioxide (MoO3) as a catalyst has been developed. However, the yield of this method is low, and it is not easy to isolate and purify the resulting L-ribose from the reactants (see: JP 11-12,294). Further, an American researcher has developed a new type of process for preparing L-ribose from D-ribose (Formula III), that is, a process wherein C1 aldehyde group of D-ribose is reduced and C5 hydroxy group is oxidized to give L-ribose (see: the following Scheme 3). But, the problems thereof such as extremely low reaction temperature of xe2x88x9278xc2x0 C., the nasty smell of sulfides, etc. should be solved first for the process to be industrially applied (see: WO 98/39,347). 
For the purpose of overcoming the problems encountered in the synthetic methods as explained above, a Japanese researcher has designed a process for preparing L-ribose using a specific enzyme (L-ribose isomerase). This process proceeds considerably smoothly due to the use of an enzyme. However, it also has the problems to be solved for its industrial application. That is, economical and stable supply of the starting material ribitol has to be secured (see: WO 99/61,648).
Thus, the present inventors, who were keenly aware of the problems, have extensively studied to develop an economical and convenient process for obtaining L-ribose under the recognition of the absolute need thereof. As a result, we have identified that such a purpose can be achieved by a process as explained below, and then completed the present invention.
Therefore, an object of the present invention is to provide a process for effectively preparing L-ribose.
The present invention relates to a process for preparing L-ribose of formula (1) characterized in that
(a) a compound of the following formula (3): 
xe2x80x83wherein R1, R2, R3 and R4 independently of one another represent C2-C6-carbonyl, benzoyl or benzyl, or R1 and R2, or R3 and R4 may combine with each other to form isopropylidene or cyclohexylidene, is reacted with a secondary amine of the following formula (4): 
xe2x80x83wherein Rxe2x80x2 and Rxe2x80x3 are identical with or different from each other and independently of one another represent straight-chain or branched C1-C6-alkyl, or may combine together with the nitrogen atom to which they are attached to form 4- to 7-membered saturated heterocycle, and then reacted with a sulfonyl group-containing compound of the following formula (5): 
xe2x80x83wherein Rxe2x80x3xe2x80x2 represents straight-chain or branched C1-C6-alkyl, phenyl or tolyl, and L represents reactive leaving group, preferably halogen or 
to give a compound of the following formula (6): 
xe2x80x83wherein R1, R2, R3 and R4 are defined as above, which has a converted chirality at 4-position carbon as compared with the compound of formula (3);
(b) the compound of formula (6) is reduced by DIBAL-H and then reacted with a compound of the following formula (7):
RXxe2x80x83xe2x80x83(7)
xe2x80x83wherein R represents benzyl, benzoyl, C1-C4-alkanoyl or C1-C4-alkyl, and X represents halogen, to give a compound of the following formula (8): 
wherein R1, R2, R3, R4 and R are defined as above;
(c) the compounds of formulae (6) and (8) are reacted with periodic acid and then reduced by DIBAL-H or NaBH4 to give compounds of the following formulae (9) and (10), respectively: 
xe2x80x83wherein R1, R2 and R are defined as above; and
(d) the compound of formula (9) or (10) thus obtained is subjected to a hydrolysis reaction in a solvent.
The process for preparing L-ribose according to the present invention can be summarized as the following Scheme 4. 
Hereafter, the process of the present invention will be more specifically explained step by step.
First, in Step (a), the 1,4-lactone compound of formula (3) is reacted with the secondary amine of formula (4) and then reacted with the sulfonyl group-containing compound of formula (5) to convert the chirality of 4-position carbon. As the similar chirality conversion process known in the art, there can be mentioned a process wherein a 1,5-lactone compound is reacted with benzyloxyamine (a primary amine) and then subjected to Mitsunobu reaction to convert the chirality of 5-position carbon in a comparatively high yield, as depicted in the following Scheme 5 (see: J. Am. Chem. Soc., 122, 2995(2000)): 
This method provides a comparatively high reaction yield. However, the starting materials such as benzyloxyamine or DEAD are expensive and thus, uneconomic; the reactant triphenylphosphine is not suitable for being used industrially; and work-up process is not easy due to some side reactions such as the formation of lactam compound by N-cyclization reaction instead of the formation of lactone compound by O-cyclization. Therefore, this method is not appropriate for being utilized in an industrial production.
Therefore, the present inventors have tried to develop a new process effectively converting the chirality of 4- or 5-position carbon in 1,4- or 1,5-lactone compound and as a result thereof, come to the conclusion that such a purpose can be achieved using a secondary amine and a sulfonyl group-containing compound.
The process of Step (a) is preferably carried out in a solvent. Any typical organic solvent which does not adversely affect the reaction may be used, but ethyl acetate, methylene chloride, tetrahydrofuran, pyridine, etc. are preferable. However, alcohol solvents are not appropriate. The reactant secondary amine of formula (4) itself can function as a solvent. Therefore, in such a case, the reaction can proceed smoothly without any need to use a separate solvent. The reactant secondary amine of formula (4) is used in an amount of 1 equivalent or more with respect to the lactone compound of formula (3). Excess amount may be used for providing an additional advantage of the reduction of reaction time with no influence on the yield. As aforementioned, since the secondary amine has the function of solvent or co-solvent in this reaction, it may be used in an excess amount sufficient to dissolve the substances in the reaction system. Among the compounds of formula (4), the preferred ones for being used in the present invention include dimethylamine, diethylamine, diisopropylamine, pyrrolidine and piperidine. The sulfonyl group-containing compound of formula (5) is used in an amount of 1 to 3 equivalents, preferably 2 equivalents with respect to the compound of formula (3). When an amount of more than 3 equivalents is used, it is not easy to remove the unreacted compound of formula (5) during the work up procedure. Among the compounds of formula (5) in the form of halide or anhydride, the preferred one is methane sulfonyl chloride. In the step of reacting the compound of formula (5), one or more selected from a group consisting of pyridine, triethylamine and dimethylaminopyridine can be used as a reaction-aid, and other auxiliaries having the same function as those mentioned above can be used. The reaction is carried out at temperatures around 0xc2x0 C., preferably at temperatures ranging from xe2x88x925xc2x0 C. to 30xc2x0 C.
In the process of Step (a), the compound of formula (3) is reacted with the secondary amine to form an amide first and then reacted with the sulfonyl group-containing compound of formula (5) to activate the hydroxy group. The activated compound is cyclized again to form a compound having the same structure as the starting compound but the chirality at 4-position carbon of the lactone ring is converted. That is, if the chirality of 4-position carbon of the starting compound is R-configuration, it is converted to S-configuration after the reaction and vice versa. That is, according to the present invention, the product compound does not have a fixed chirality but have a relatively converted chirality at 4-position carbon compared with the starting compound. The total reaction yield of this process is as high as 85% or more.
In Step (b), the compound of formula (6) is reduced by DIBAL-H (diisopropylaluminum hydride) and then reacted with the compound of formula (7) to give the compound of formula (8). The reaction is carried out at temperatures ranging from xe2x88x9278 to xe2x88x9230xc2x0 C., preferably at xe2x88x9250xc2x0 C. DIBAL-H and the compound of formula (7) are used in amounts of 1 equivalent and 1 to 2 equivalents (preferably 1.2 equivalent), respectively, with respect to the compound of formula (6). The reaction of DIBAL-H is preferably carried out in a solvent, and as the examples thereof one or more selected from a group consisting of toluene, methylene chloride and tetrahydrofuran can be mentioned. The step of reacting with the compound of formula (7) proceeds in a solvent such as tetrahydrofuran (when R=benzyl, etc.) or pyridine (when R=benzoyl, etc.) and optionally in the presence of a reaction aid such as potassium hydroxide or 18-crown-6.
In Step (c), the compounds of formulae (6) and (8) are oxidized by periodic acid and then reduced by DIBAL-H or NaBH4 to give the compounds of formulae (9) and (10), respectively. The reaction of the compound of formula (6) or (8) with periodic acid results in the quantitative formation of an aldehyde compound. When the aldehyde compound obtained from the compound of formula (6) is reduced by DIBAL-H in an amount of 2.0 equivalents or by NaBH4 in an amount of 0.5 to 1.0 equivalent, preferably 1.0 equivalent with respect to the compound of formula (6), the alcohol compound of formula (9) is given in a yield of 60 to 95%. When the aldehyde compound obtained from the compound of formula (8) is reduced by DIBAL-H or NaBH4 in an amount of 1.0 equivalent with respect to the compound of formula (8), the compound of formula (10) is given in a high yield of 90% or more. The reference (J. Org. Chem., 61, 5178(1996)) may be referred to the specific reaction conditions.
Finally, in Step (d), the compound of formula (9) or (10) obtained in Step (c) is subjected to hydrolysis reaction in a solvent in the presence of an acid or a base to give L-ribose of formula (1). Solvents which can be used include water, dioxane, methanol, ethanol, and mixtures thereof. Reaction temperature and time of Step (d) reaction may be different depending upon the acid or base used. For example, the reaction is carried out for 24 hours at about 40xc2x0 C. when hydrochloric acid is used, and reflux condition is required when trifluoroacetic acid is used. More specifically, L-ribose of formula (1) can be obtained by hydrolyzing the compound of formula (9) in the presence of an acid, or by hydrolyzing the compound of formula (10) either in the presence of a base and then in the presence of an acid or in the presence of an acid from the first. The specific method to be applied may be selected depending on the kind of substituent R. For example, when the substituent R is benzoyl, it is preferable to select a method wherein hydrolysis reaction is carried out in the presence of a base and an acid by turns. When the substituent R is other than benzoyl, an acid hydrolysis method may be selected. The acid which can be used in Step (d) includes hydrochloric acid, sulfuric acid and trifluoroacetic acid, and the base includes sodium hydroxide and potassium hydroxide. The base or acid is used in a catalytic amount wherein the catalytic amount means about 0.1 equivalent.
L-ribose obtained by the method as explained above may be purified by recrystallization in a solvent or by silica gel column chromatography.
The present invention will be more specifically explained in the following examples. However, it should be understood that the following examples are intended to illustrate the present invention but not in any manner to limit the scope of the present invention.